Fluid process systems typically use valves such as, for example, rotary valves to control the flow of process fluids. In general, rotary valves typically include a fluid flow control member disposed in a fluid path and rotatably coupled to the body of the rotary valve via a shaft. Typically, a portion of the shaft extending from the rotary valve is operatively coupled to an actuator (e.g., a pneumatic actuator, an electric actuator, a hydraulic actuator, etc.), which operates the flow control member. To couple the actuator to the valve shaft, a lever or lever arm is typically employed. The lever converts a linear displacement of an actuator stem into a rotational displacement of the valve shaft. Thus, rotation of the lever causes the valve shaft and the flow control member (e.g., a disk, a ball, etc.) to rotate to increase or restrict the flow of fluid through the valve. In operation, a controller may be used to control the displacement of the actuator to rotate the lever and the valve shaft and, thus, the flow control member of the valve to a desired angular position to achieve a desired fluid flow through the rotary valve.
However, shaft couplings such as, for example, levers, that convert linear translation into rotational movement of a valve shaft are often prone to backlash. Backlash, which occurs if the lever is not properly sized to the shaft, and leaves clearance between contacting surfaces of the lever and the shaft which results in lost motion and reduced accuracy of fluid flow control through the valve. Additionally, Industry standards (e.g., International Organization for Standardization) may require an actuator to couple to differently sized valve shafts. Adherence to the ISO standard requires that actuators and valves made by multiple or different manufacturers can be interchangeably coupled to each other without requiring modification of the actuators or the valves. To substantially reduce backlash from inaccurately sized couplings and to facilitate the compatibility of control valves with a variety of actuators, many available actuators have shaft couplings such as, for example, a lever adapted with a collet to receive a valve shaft. In particular, many off-the-shelf actuators provide collets having a square bore or opening to receive differently sized square valve shafts.
However, to prevent lost motion from occurring between the lever and the square valve shaft, the collet must provide sufficient clamping force to the square end of the valve shaft. Failure to provide a sufficient clamping force between the collet and the valve shaft typically results in a loose mechanical coupling and, thus, lost motion between the lever and the valve shaft. Such lost motion may lead to inaccurate positioning of the flow control member and, thus, poor control over the fluid flowing through the valve.